1. Field of the Invention
This invention relates to an image pick-up system, and more particularly to an image pick-up system which uses, as image pick-up means for converting an optical image into an electrical expression, a signal accumulation type image pick-up or image scanning element known as a CCD, BBD, or MOS image sensor.
2. Description of the Prior Art
Generally, in a signal accumulation type solid state image pick-up element such as a CCD, a dark current is dominant as the noise component for a signal obtained over a long accumulation time. That is, the dark current increases in proportion to the accumulation time of the signal, which leads to the disadvantage that the substantial sensitivity of the element decreases with the accumulation time of the signal.
Heretofore, an attempt has been made to solve that problem by presetting the allowable longest signal accumulation time to a shorter time, but this in turn has given rise to the new inconvenience that the limit of the brightness or luminance of the light that can be picked up is lower than what would be desirable.
Also, when the element is used at a relatively high temperature, the dark current comes into the signal as a non-negligible noise component, which also has led to the disadvantage that the dark current increases as the accumulation time becomes longer.